1. Field of the Invention
The present invention relates to a control system for a DC power transmission system comprising a rectifier station, an inverter station, an AC network for each station, a DC line connecting said stations together, control means for the system comprising a current control circuit for the transmission system which delivers a common current order to a current regulator for each station, and a circuit in one of said stations for setting a current margin between the said two stations.
Such DC transmission systems are used for the asynchronous interconnection of AC networks. The networks may be located at a distance from each other, for example separated by watercourses or sparsely populated areas, or they may be located close to each other in which case the rectifier and inverter stations are usually built together. The latter case is referred to as a back-to-back connection, and is useful if it is desirable to avoid any synchronous connecting of the two networks together.
2. Description of the Prior Art
For control of a DC transmission system as described above, it is usual to control the rectifier and inverter stations from a current regulator located in each station. These current regulators are in turn, controlled by a common current order which can be derived from some other control parameter. By introducing a so-called current margin into the control system of one of the stations, the rectifier station can have a greater resulting current order than the inverter station, which results in the station with the highest voltage setting, normally the rectifier station, determining the transmitted direct current and in the other station determining the direct voltage. In this way--as is described in Uhlmann: "Power Transmission by Direct Current", Springer-Verlag, 1975, pages 128-129--stable operating conditions in the transmission are obtained.
In addition, the control principle described above offers the advantage that it is relatively simple to introduce various auxiliary parameters into the basic control system in order to increase the capability of the system and thus make allowance for variations and disturbances in the AC networks or in the DC transmission line.
Among other things, it has been found that in the event of voltage drops in the power-receiving AC network, i.e. the network of the inverter station (for example caused by short-circuits, ground faults or the like), the transmitted direct current should be reduced in order to reduce the power transmitted to this network, thus ensuring the stability of this network. Such a current reduction suitably takes place by introducing a current-order limiting circuit in each station. Such a limiting circuit is of general value to put an upper limit to the current order which dictates the transmission, for example from some superordinate control system. Said limiting circuit is then made dependent on the voltage, and since a current limitation must take place at the same time in both stations to ensure the stability of the transmission system, it is a natural thing to start from a voltage which is easily accessible in both stations, namely the DC voltage on the transmission line.